The present inventors have studied air flows caused while a fan is rotating since fan noises are generated by air flows particularly when the latter are disturbed irregularly. Upon observation of air flows during rotation of a conventional fan with the aid of a styrene particle method, it was found that no main air flow is present over a considerable area at the tip of each blade of the fan, the main air flow being defined as an air flow (indicated at U in FIG. 4) in an axial direction of the fan while the latter is rotating. An oil film method was used to observe air flows along the surfaces of fan blades. As a result, it was also found that air flows mainly in a radial direction (shown at R in FIG. 5) at the tip of each blade of the prior fan. On the basis of these experiments, the present inventors have concluded that conventional fans produce noises due to air flow distrubances at the tip of each fan blade. To confirm such conclusion, air flow disturbances during rotation of a fan were measured by a hot wire anemometer, and main air flow distribution was measured by a three-hole Pitot tube. The results of such measurements indicated that air flow is greatly disturbed and gets stalled at the blade tip. The foregoing conclusion was therefore justified.
An air flow at the tip of each fan blade is considered to be disturbed by the following reasons: As shown in FIG. 1, air is believed to flow in streams a around the blade tip 1e from front to rear surfaces of the blade. It is also considered that air is caused to flow from a base end portion 1g to the tip 1e of each blade 1a as indicated at b in FIG. 2. Where the fan 1 is used with a shroud 3 as illustrated in FIG. 3, the blade tip 1e traverses a boundary layer on a wall of the shroud, causing air flow disturbances at the blade tip 1e.
U.S. Pat. No. 3,914,068 (corresponding to West German Pat. No. 2203353) discloses a cooling fan for automobiles. The known fan has blades, each including a tip or distal end inclined at an angle greater than that at which a proximal end of the blade is inclined, an arrangement which will deliver a sufficient amount of air to an internal combustion engine even when the latter rotates at low speeds.
A fan according to the present invention is of the type driven by an electric motor at a constant output thereof. Where the prior fan having a greater blade angle at the blade tip than at the proximal end is to be driven by a motor having a constant output, the absolute value of the blade angle at the proximal end is required to be considerably small. This is because, with the greater blade angle at the blade tip, energy from the motor consumed at the blade tip is increased and energy consumed at the proximal end is reduced accordingly. It has been known that if the blade angle at the proximal end of the blade is too small, no effective work is done at the proximal end and the proximal end of the blade causes air flow to be disturbed producing noises at an increased sound pressure level. Therefore, the prior fan as a whole cannot reduce noises to a large extent since the proximal ends of the blades produce larger noises, though noises are slightly reduced at the blade tips.